1. Field of the Invention
The present invention relates to a switch for use in a dielectric line provided for the propagation of an electromagnetic wave such as a millimeter wave, the present invention also relates to an antenna device employing the dielectric line.
2. Description of the Related Art
Conventionally, with a vehicle radar module and a radio communication module, there had been suggested a circuit which is in the form of non-radial dielectric line (hereinafter, referred to as NRD GUIDE). In practice, such NRD GUIDE may be obtained in the following way. Namely, some units such as directional coupler or isolator may be easily fabricated by bringing dielectric lines into mutually adjacent positions and by adding some additional substances such as ferrite, then, a planar circuit board is inserted into a central position of the dielectric lines so as to attach semiconductor elements and some other functional elements in positions, thereby forming the NRD GUIDE.
FIG. 38A is a partially sectional side elevation illustrating the structure of a millimeter wave radar module using the NRD GUIDE. FIG. 38B is a plane view illustrating the radar module of FIG. 38A. In fact, the radar module is equipped with the NRD GUIDE which is for use as a propagation path for a millimeter wave to pass therethrough. Here, the NRD GUIDE itself includes an upper conductive plate, a lower conductive plate, linear or curved rod-like dielectric strips interposed between the upper and lower conductive plates. In more detail, as shown in FIG. 38B, the radar module further includes an oscillator (millimeter wave oscillator), an isolator, a coupler (directional coupler), a circulator, a mixer, a primary radiator for signal transmission and signal reception. Further, a dielectric lens is installed above the primary radiator by a predetermined distance.
If the radar module shown in FIGS. 38A and 38B is used as FM-CW radar which employs a transmission signal (which has been treated in frequency modulation so as to become a CW (Continuous Wave), a millimeter wave signal generated in the oscillator and treated in FM modulation, is first passed through the isolator and then through the coupler. Afterwards, one half of the signal is supplied to the circulator, while the other half of the signal is used as a local signal to be supplied to the mixer. The signal supplied to the circulator is transmitted to a dielectric resonator of the primary radiator, passing through an electromagnetic wave radiating window so as to be radiated from the dielectric lens. Then, a reflected wave from an object is incident on to the dielectric lens, received by the primary radiator (including an electromagnetic wave radiating window and a dielectric resonator), and further passed through the circulator so as to be supplied as a RF (Radio Frequency) signal to the mixer. In the mixer, the RF signal and the local signal are mixed together, to produce an output signal as an IF (Intermediate Frequency) signal containing a distance information and a speed difference information.
In the past, a monitoring radar module (mounted on a vehicle for monitoring a forward situation) is provided with a beam antenna having a highly sensitive directivity so that it has a high gain and can prevent any possible interference from a vehicle travelling along an adjacent line. However, when a vehicle is travelling along a curved line, there will be a detection mistake as if a vehicle running along an adjacent line is travelling ahead of itself. In order to solve this problem, not only is it necessary to obtain a distance information indicating a distance between itself (this vehicle) and an ahead running vehicle, but also it is necessary to obtain an azimuth information indicating the azimuth of a vehicle travelling along an adjacent line.
Conventionally, there have been two methods which can be used to obtain an azimuth information. One method employs a scanning type radar capable of rendering an electromagnetic wave beam to scan within an appropriate angle. The other method employs a mono-pulse type radar which functions with the use of a sum signal obtained by adding together signals from two or more antennas of different radiating patterns, and also with the use of a deference signal obtained by performing a deducting calculation among the signals from the two or more antennas of different radiating patterns.
With the above scanning type radar, it is allowed to mechanically rotate the radar module by a motor to enable the radar beam to scan within a range of a sector (a fan shape), but it is difficult to perform a high speed scanning, and the apparatus as a whole is too large and bulky. Although it is possible to provide an electronic switch within the circuit to perform a desired change-over among a plurality of antennas, it is still needed to use many antennas and a highly functional NRD GUIDE switch. As a result, it is difficult to manufacture the scanning type radar with a compact size and a low cost. Further, if using a different manner where a beam is caused to perform a desired scanning but without moving the antennas, it is possible to perform a phase scanning capable of changing a directing angle into any direction by arranging the antennas in a predetermined array and by controlling the phase of a feeding signal (which is to be fed to the antennas). However, there still exists a problem that it is difficult to manufacture the scanning type radar with a compact size and a low cost.
On the other hand, with the above mono-pulse type radar, the apparatus is allowed to be made compact in size. However, since it is needed to cover an azimuth range (which is to be detected), it is necessary to use antennas each having a large beam width. For this reason, the gain of the radar is correspondingly reduced. To solve this problem, it is required to either increase an output power of the radar in order to effect a needed detection on a position located far away, or to provide active functional element for use as an amplifier in a signal receiving circuit so as to improve its signal receiving sensitivity. However, at present time it has been proved difficult to obtain a desired effect from the provision of the active functional element if a signal is in the form of a millimeter wave.